Question: $ \left(\dfrac{81}{49}\right)^{-\frac{1}{2}}$
Explanation: $= \left(\dfrac{49}{81}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{49}{81}$ Figure out what goes in the blank: $\Big({\dfrac{7}{9}}\Big)^{2}=\dfrac{49}{81}$ So $\left(\dfrac{81}{49}\right)^{-\frac{1}{2}}=\left(\dfrac{49}{81}\right)^{\frac{1}{2}}=\dfrac{7}{9}$